Method of treating molten metal under vacuum



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METHOD OF TREATING MOLTEN METAL UNDER VACUUM Filed Jan. 20, 1964 2 Sheets-Sheet 1.

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C. W. FINKL Aug. 22, 1967 METHOD OF TREATING MOLTEN METAL UNDER VACUUM Fi led Jan. .20, 1964 2 Sheets-Sheet United States Patent 3,337,329 METHOD OF TREATING MOLTEN METAL UNDER VACUUM Charles W. Fink], Chicago, Ill., assignor to A. Finkl & Sons Company, Chicago, [1]., a corporation of Illinois Filed Jan. 20, 1964, Ser. No. 340,594 14 Claims. (Cl. 75-49) This invention relates to vacuum degassing molten metals such as steel and particularly to a method and apparatus therefor, and is a continuation-in-part of my co-pending application Ser. No. 54,745, filed Sept. 8,- 1960, now abandoned and my co-pending application Ser. No. 777,664, filed Dec. 2, 1958, now Patent No. 3,236,635.

A new method of refining molten metals, particularly steel, has been developed in recent months. In this method, a ladle of molten steel is placed in a vacuum tank and subjected to a very low vacuum. The vacuum may, for example fall to absolute pressures on the order of 50 to 500 microns of mercury at certain times in the cycle. A purging gas is admitted to the ladle near its bottom and bubbled upwardly to the top of the melt where it escapes at the surface at the same time that the metal is subjected to the very 'low vacuum. The upward passage of the purging gas sets up a mechanical stirring action which brings substantially undegassed molten metal from remote portions of the ladle to the surface where the vacuum can act on it. This mechanical stirring action is in contrast to a chemical reaction occurring with gases admitted to molten metal in desulphurizing operations.

The object of subjecting the molten metal to a vacuum and simultaneously bubbling a purging gas upwardly through it is to remove the included deleterious gases, including oxygen, nitrogen, and, particularly, hydrogen. As is well known in the art, hydrogen is very deleterious in finished steel. Failures can quickly occur in die blocks, large forgings, and the like, if the included hydrogen gas content is too high.

By subjecting the molten metal simultaneously to a vacuum and a purging gas, the included gas content can be carried down to a value far below the values obtained by the best of the prior art methods, The hydrogen content for example can be readily and consistently reduced to less than 2.2 p.p.m., which, in the trade, is commonly known as the upper tolerable hydrogen limit for aircraft quality steel. The hydrogen content can quite consistently be reduced to 1 ppm. or below.

During the upward travel of the purging gas, some of the included deleterious gases in the molten metal in all probability migrate into the upwardly traveling bubbles. When the purging gas bubbles reach the surface, the included gases which have migrated into the bubbles are then drawn off through a suitable vacuum system along with the purging gas.

The primary effect of the purging gas, however, is to set up a mechanical stirring action in the ladle, or other container holding the molten met-a1. Thus, a circulatory motion within the metal is set up which brings undegassed metal from regions remote from the surface up to the surface where the vacuum in the vacuum tank acts on it.

It is important during the process to closely control the degree of vacuum. It has been found, contrary to the teachings of well known workers in the art, that commercially feasible in-the-ladle degassing is possible if a circulatory motion is set up and the vacuum is carried down to sufiiciently low absolute pressure values. In the preferred operation of the invention, the vacuum is carried down to a value of around 2 millimeters of mercury, and, even more preferably, to values on the order of about 1 millimeter of mercury and below. In commercial operation, it has been found that vacuum values in a range having a lower 3,337,329 Patented Aug. 22, 1967 limit of approximately 300 to 500 microns of mercury give excellent results, though values as low as 5 to 50 microns are presently the subject of activity.

Heats of molten metal varying from 2 to over tons have been degassed at only a nominal cost per pound utilizing the above described method. It may well be, depending upon the practice of the shop in which the metal is treated, that the over-all cost of producing steel in ingot or semi-finished form is reduced by the use of this process. This is possible because the fine quality resulting from the combined purging and vacuum operations may eliminate one or more time-consuming heat-treat operations,

The process, and variations of it, and apparatus for carrying out the process are further illustrated (in addition to the aforementioned applications), :in my co-pending United States applications Ser. No. 186,569 filed Mar. 6, 1962, which in turn is a divisional application of application Ser. No. 855,442 filed Nov. 25, 1959, now Patent No. 3,084,038, and Ser. No. 176,493 filed Feb. 26, 1962, now Patent No. 3,236,636, which is in turn a divisional application of application Ser. No. 27,826 filed May 9. 1960, now Patent No. 3,071,458,

This application is a continuation-in-part of the aforesaid applications and covers improvements in the method of treating molten metal as disclosed in my prior applications and structure for performing the improved method.

Experience has shown that when molten metal is purge'd through the bottom of the ladle or through the stopper rod, foreign material is not always completely removed from the depression formed in the nozzle block. This is because the circulation within the ladle sometimes has no effect in this depression. Inclusions of materials may therefore be trapped in the nozzle block depression and remain there during the entire degassing operation. When the metal is teemed into an ingot mold, these foreign inclusions pass into the mold and are therefore present in the ingot.

In all probability the metal does not solidify in the nozzle block depression, but a slush is unquestionably formed. This slush is sufilcient to entrap inclusions.

Present methods and apparatus for practicing the vacuum purging process discussed above require some special equipment. Thus, when the purging gas is admitted to the molten metal through the stopper rod, a specially designed rod having a hollow interior must be used. If a special purging disc is used, the disc, as well as the rod refractory, must be replaced after each heat since its service life is only one heat.

It has also been found that in an installation in which gas is admitted to the melt through the stopper rod, the rod sleeves occasionally tend to separate during operation. This is due to the vibrations set up by the upward passage of the bubbles of purging gas. Should a complete separation occur, the stopper rod will become inoperative or the molten metal may begin to leak out of the ladle.

As is well known in the art, metal which is too cold in the ladle may cause the blackhead to freeze to the nozzle. Where this condition occurs, it is impossible to pull the rod up to teem from the ladle. An oxygen lance must be used to burn the solidified metalaway, or a pricker bar used to force the blackhead up from below.

As is also well known in the art, a shanker may form when the metal is too cold in the ladle. In this specification, a shanker will be used to denote a condition in which a solid or semisolid glob of metal lodges in the nozzle opening or hangs downwardly therefrom, much like an icicle. Whenever a shanker is formed, it restricts the flow, and may lengthen the teeming time which is a disadvantage since time is of the essence.

If the shanker hangs downwardly, tongs are often used to remove it. Removing a shanker by any procedure is a dangerous operation, extremely wasteful of time, and could seriously restrict the flow of metal.

Further, if the shanker is not completely removed, a loose pouring stream results. A loose pouring stream is undesirable because there is an opportunity for hydrogen and other deleterious gases to reenter the steel from the atmosphere as it is teemed, thereby partially nullifying the effect of the degassing. The occurrence of any one or more of the above-mentioned conditions may well result in erratic pouring behavior of the degassed metal.

It is also important that the surface of the melt be at least substantially slag free, and preferably entirely slag free, during at least a substantial portion of the time it is exposed to the vacuum. The presence of slag on the surface of the melt, particularly at the commencement of exposure to vacuum, has several deleterious effects, perhaps the foremost of which is hindrance to the degassing action it presents. The weight of the slag prevents exposure of the surface of the melt to the vacuum, and, further, the weight of the slag may have the effect of decreasing the vacuum at the melt surface.

Accordingly, an object of this invention is to provide a means of vacuum degassing, with or without the use of a purging gas, which maximizes exposure of the melt to the effect of the vacuum.

Another object of this invention is to provide a method and means for flushing foreign material out of the pocket block during vacuum degassing process whereby only clean liquid metal is finally poured into the ingot mold.

Yet another object is to provide a. flushing system for use in a vacuum degassing process which utilizes standard stopper rods, eliminates large purging discs, and raises no special servicing problems.

A further object is to provide, in a vacuum degassing system, a quickly and easily replaceable purgant admission assembly, said assembly comprising a replaceable nozzle having incorporated therein a purging disc so that the disc is renewed each time the nozzle is replaced.

A further object is to provide a gas purging system in which the gas emission plug may be used for several heats and often need not be replaced until the ladle lining is renewed.

Yet another object is to provide a means for and a method of admitting purging gas to molten metal under vacuum which prevents separation of the stopper and sleeve due to vibration of the rod.

Yet another object is to provide a means for and a method of preventing the formation of shankers.

Another object is to provide a means for and a method of preventing freezing of the blackhead in the ladle nozzle.

Yet another object is to provide a method of degassing (as contrasted to desulphurizing) molten metal by subjecting a batch of molten metal to a vacuum and simultaneously bubbling an inert purging gas upwardly through the metal from the lowest point in the ladle to thereby provide a maximum mechanical stirring effect.

Yet another object is to provide a method of degassing molten metal under vacuum by bubbling an inert mechanical purging gas upwardly through the molten metal which utilizes vacuum pressure values of a very low magnitude.

Yet another object is to provide a method of vacuum degassing molten metal which is applicable to a wide range of sizes and heats.

Yet another object is to accompish all of the aforementioned described objects at substantially no increase in operating cost.

Other objects and advantages of the invention will become apparent upon a reading of the following description of the invention.

The invention is illustrated more or less diagrammatically in the accompanying drawings, wherein:

FIGURE 1 is a view of a general assembly illustrating one mode of carrying out the present invention;

FIGURE 2 is a view, to an enlarged scale, of the nozzle block structure of FIGURE 1; and

FIGURE 3 is a view similar to FIGURE 2 showing another embodiment of the invention.

Like reference numerals will be used to refer to like parts throughout the following description of the drawings.

In FIGURE 1, a vacuum tank is indicated generally at 1 resting on a plurality of supports, such as I beams 2, on the floor 3 of a pit 4. The vacuum tank 1 consists of an upper domed section 5 and a lower section 6. The sections are sealed as at 7 and one or more ports 8 may be formed in the dome to provide a view of the inside of the tank.

A ladle for containing molten metal is indicated generally at 10. The ladle rests on a support ring 11 which rises upwardly from the bottom 12 of lower section 6. A layer of refractory 13 protects the bottom of the tank from excessive heat and spillage.

Ladle 10 may be of conventional construction. In this instance it includes a stopper rod 14 which terminates in a blackhead 15 seated in a nozzle block structure 16, which will be described in detail hereinafter. The stopper rod is raised and lowered by any suitable elevating mechanism indicated generally at 17. In this instance, the stopper rod is shown as composed of a plurality of refractory sleeves 18.

The blackhead 15 and nozzle block structure 16 of FIGURE 1 is illustrated in detail in FIGURE 2. The nozzle block structure is composed essentially of an upper block 21 and a lower block 22. The blocks are so designed as to be slidably received within an opening 24 in the bottom 25 of the ladle.

Each of the blocks 21 and 22 is formed with a center aperture which forms a nozzle receiving aperture 26. A conventional nozzle 27 having a discharge passage 28 is received in nozzle aperture 26. Any suitable packing material such as a refractory mortar forms a tight seal between the nozzle and aperture to prevent leakage of molten metal through the nozzle block. Upper block 21 is countersunk to provide a downwardly, inwardly tapered depression or cavity 29.

The lower end of the nozzle is tapered as at 30 so as to be snugly received within a nozzle plate assembly 31. The nozzle plate assembly includes a lower disk 32 which is apertured in line with the passage 28. A sand plate 33 is held snugly against the bottom of lower disk 32 by dogs 34 which are slotted so as to permit the sand plate 33 to slide in a horizontal plane. It will be understood that during operation the nozzle is filled with sand 35 so that should the stopper rod fail, the molten metal will not flow out. The nozzle plate assembly 31 is held snugly against the bottom of the ladle by a pair of studs 36 and wedges 37.

An inert purging gas is admitted to the molten metal in the ladle during the degassing cycle by the system illustrated best in FIGURES l and 2. The source of inert purging gas is indicated generally at 40. Any suitable carrier agent or inert gas such as argon or helium may be utilized. It is considered that other carrier agents or inert gases including neon, krypton, xeon and radon might be utilized and the terms carrier agent and inert gas are intended -to include these gases. Surprisingly, dry air has been successfully employed. Since no chemical reaction takes place between the purging gas and the molten metal, almost any gas which is inert with respect to the molten metal being treated may be utilized. The purpose of the purging gas is only to set up a mechanical circulation within the ladle, and it does not react with the molten metal in any way. The gas passes through a suitable valve 41 and a flexible hose 42 to a quick-disconnect junction 43 at the tank wall. A gas conduit 44 secured to the side of the ladle is adapted to be connected .to the junction 43 when the ladle is placed within the vacuum tank. The conduit 44 extends downwardly to the bottom of the ladle and curves under the bottom, as at 45. A T 46 is provided for maintenance and testing of the purging plug. From T 46, the pipe passes upwardly through the bottom of the ladle, as at 47, and then angles upwardly, as at 48, to a porous refractory plug 49. The plug 49 has a porosity sufficient to permit purging gas to flow outwardly into the ladle while preventing inward penetration of the molten metal. As can be clearly seen in this figure, the exposed face of the purging plug is flush with the depression 29 in upper block 21. A suitable washer 51 Welded to the angling section 48 of the gas pipe s-teadies the pipe. It will be understood however that the washer can be dispensed with.

The quickdisconnect junction 43 is preferably of the type in which a valve is located in both ends of the junction, and the valves are closed unless engaged. It is therefore possible to have all inert gas flow devices set to the on condition, and the instant the connection is made, purging gas begins to floW. As a consequence, little or no purging gas is wasted. Standard piping and connections may be used in the gas admission system.

In FIGURE 3, a slightly different arrangement is illustrated. In this instance, gas pipe 45 extends straight upwardly as at 54 and terminates in a porous refractory purging plug 55. A second plug 56 whose upper surface is flushed with the upper surface of ring 60 of pocket block structure 61 is located immediately above plug 55. Lower block 62 is apertured to receive the upwardly extending section 54 of the gas pipe.

It will be noted that no taper is needed to maintain the plugs in place. This is due to the fact that only a very small opening is needed to admit the gas to the melt and the outward pushing force of the gas on the plug is not sufiicient to overcome the frictional holding effect of the plug aperture. Installation of the plug is much quicker as contrasted to known systems utilizing porous plugs. Alternately, refractory cement may be used if desired.

The use and operation of this invention is as follows:

By utilizing the structure and method of this invention, it is possible to remove approximately two-thirds of the hydrogen, at least one-half of the oxygen, and at least one-fourth of the nitrogen included in the molten metal at the time it leaves the furnace. A typical degassing cycle, in this case a double slag basic process, is described below. It should be understood however that the method and apparatus of the invention may be utilized with any slagging process, basic or acid, single or double slag, so long as the molten metal is capable of being degassed.

A furnace is charged with suitably selected scrap. Generally, alloy steels will be made in an electric or open hearth furnace, but the invention is not confined to any particular type of furnace or any particular procedure.

The charge is melted until the metal is completely molten. This may take anywhere from three to three and one-half hours for a charge of approximately 35 tons in an electric furnace. During melting, oxygen, iron ore, mill scale, or other oxygen bearing compound is admitted to the steel to oxidize the carbon, and burn down scrap.

Towards the end of the melting operation, the slag on top of the molten metal is rabbled off. This may be done in any convenient manner such as by use of a wooden hoe. This slag serves as an insulation to reduce heat losses and also acts as a protective blanket which prevents the molten steel from exposure to the atmosphere. The slag also contains several constituents which are the result of chemical reactions in the bath and at the interface in addition to the slag forming materials originally charged into the furnace. Since the exact chemical reaction which take place are not part of the invention, they are not described in detail.

After the oxidizing slag has been removed, a reducing or deoxidizing slag is added. This slag serves, among other things, to reduce heat loss.

It will be understood that during and after the oxidizing and reducing slag operations, tests will be run to determine the alloy content of the metal.

After all desired alloy additions have been made in the furnace, and the heat is ready to be tapped, most of the reducing slag is rabbled 01f. It is important that only a very bare minimum of slag, or, in other words, a substantially decreased slag cover, be retained on the surface of the metal. If a heavy slag, such as the type used in a desulphurizing operation, is retained, the degassing is hindered. If the slag is entirely removed the degassing will proceed in excellent fashion, but there may be too much heat loss, for example, during transfer from the furnace to the vacuum tank, and too much oxidation. The amount of heat lost will depend to a considerable extent on the procedure and handling skill of the operators, and it is possible under certain conditions for transfer from furnace to tank to be so fast that heat loss is minimal. As a pratical matter it is difficult, though not impossible, to completely remove the slag cover without letting the metal cool too much. For all practical purposes the metal may be considered as uncovered by slag when it is tapped from the furnace into the ladle.

There are other reasons why the slag should be su stantially entirely removed before the furnace is tapped. For one thing, the gas in the slag may overload the vacuum system. The greatest volume flow rate of gas occurs at the beginning of the cycle since the vacuum acts over the entire surface of the ladle. The gas in the slag may be suflicient to overload the vacuum system at the start of the cycle whereas later in the cycle when the volume flow rate decreases the slag gas may be more readily handled.

Secondly, maintenance is reduced. The boil is most violent at the start of the cycle and slag on the surface of the ladle at this time could be blown all over the vacuum chamber by the boiling action.

Thirdly, the danger of explosion is reduced. Slag usually contains manganese which may disassociate into a slag dust. Since oxygen is present in the tank at commencement of degassing, it is possible that just the correct mixture of manganese dust and oxygen will occur to produce an explosion. At the end of operations, the oxygen is almost entirely removed, and the tendency for the slag to disassociate and an explosion to occur is considerably reduced.

Fourthly, slag under vacuum attacks refractories.

Before the ladle is actually placed in operation a source of compressed air is connected to T 46. If air comes through the purging plug, which condition can be 'detected by placing a hand over the plug, the operator knows the plug is unobstructed.

After the molten metal is tapped from the furnace into the ladle, the ladle is transferred to the vacuum tank and the cover swung over the tank.

In view of the fact that for all practical purposes the surface of the metal is directly exposed to the vacuum, it may be convenient to provide a radiation shield which acts to re-radiate heat from the molten metal back into the ladle. As soon as the dome 5 is swung into place and sealed on the lower section 6, the vacuum pumps are turned on to evacuate the vacuum chamber through outlet 71. In several commercial installations it has been found convenient to utilize a four-stage steam ejector vacuum pump system. A five, or even asix-stage system may be employed if desired.

As mentioned above, purging gas passes into the melt the instantthe connections are completed at junction 43. Another advantage of the illustrated gas system is that the pressure in the conduit before connection tends to resist penetration of the molten metal into the purging plug. Likewise, when the gas is shut off preparatory to teeming, the gas in the conduit continues to resist penetration of the plug by the metal.

In one commercial installation a high temperature refractory mortar has been substituted for the upper plug 56 of FIGURE 3. This is especially helpful when the plug is too porous and the metal tends to penetrate it. The mortar protects the plug until the gas pressure is turned on at which time it is blown off by the gas and carried to the surface by the gas bubbles. The vacuum may be carried out for any length of time commensurate with the degassing desired and the tolerable temperature drop.

It may be convenient at some time during the degassing operation, generally towards the end, to admit additions to the ladle, either alloy additions, deoxidizers, or slag forming materials or any combination thereof. It is desirable, for example, to hold off adding aluminum, and sometimes silicon, vanadium and/ or manganese until the end of the cycle because the presence of aluminum, or other highly deoxidizing materials, retard the degassing effect. In addition, by holding off the addition of slag forming materials until the Very end of the cycle, the steam ejectors are not overloaded because the slag Will be degassed at a time when the rate of gas liberation from the steel is at a minimum.

The admission of the purging gas sets up a circulatory effect whose direction is indicated by the arrows in FIGURE 1. As the individual bubbles 72 of purging gas pass upwardly through the area adjacent the bottom of the stopper rod, they flush out any foreign materials contained in the depression 29. At the same time, a circulation is set up whereby undegassed metal from a remote area such as 73 is caused to circulate up to the surface 74 where the vacuum will act on it. Further, since the purgant rises upwardly close to the stopper rod, the slag surrounding the rod will be pushed out of contact with it, thus reducing the rate of erosion of the refractory stopper rod at the metal surface since slag, particularly under vacuum, attacks and erodes refractory much faster than molten steel. This enables a less expensive stopper rod refractory to be used.

The boil in the ladle is quite violent while purging gas is being admitted. This is due, in part at least, to the very great volume expansion of the gas. Assuming the temperature of the melt is on the order of 3000 degrees F., and the purging gas is at approximately 20 p.s.i.g., and the pressure in the tank is around 500 microns, the gas will expand approximately 9000 times its original volume. As those skilled in the art will readily appreciate the conventional tapping temperature for the type of steel described herein is about 2915 F. so it will be noted that a moderate superheat has been imparted to the steel.

When the degassing operation has been completed, the vacuumproducing equipment is shut off, the main valve in outlet conduit 71 is closed, and a non-explosive gas, such as nitrogen, is admitted to the tank. As soon as the pressure in the tank is brought up to atmospheric pressure, the tank is opened and the ladle removed. The molten metal may then be transferred to a pouring or teeming station where it is teemed directly into an ingot mold or other suitable receptacle. It will be understood that before the molten metal is actually admitted to the ingot mold, the sand 3J5 in the nozzle passage is blown out by compressed air so that it will not pass into the mold. The metal may be teemed into a mold filled with an inert gas. It may also be convenient to shroud the stream in an inert gas. It will also be understood that back filling of the tank with nitrogen and/or inert teeeming may be employed irrespective of whether or not purging is employed.

Improvement in efiiciency using block purging can be accomplished at no increase in operation cost. In fact, certain economies can be effected because it is not necessary to utilize a specially designed stopper rod. In addition, the porous refractory purging plug of this invention will last for anywhere from three to ten heats, or occasionally as long as the ladle lining. Commercial plugs cost about 45 a piece as contrasted to special stopper rod purging disks which cost six to eight dollars each and must be replaced each heat.

It has also been noted that that the most effective degassing is carried out at an absolute pressure of less than one millimeter of mercury. Experience has shown that to get commercially satisfactory results, it is necessary that the absolute pressure be on the order of about one millimeter of mercury or less, although pressure of two millimeters of mercury produce acceptable results. The exact pressures used will of course affect the time needed to effectively degas and any selection of pressures and degassing time is in effect a compromise. It is considered that little degassing can be done at pressures as high as ten millimeters no matter how long the melt is held in the vacuum tank. Pressures on the order of four to six millimeters of mercury do not give as satisfactory results as do pressures of two millimeters of mercury or below.

Although the invention has been illustrated and described in connection with certain practical embodiments, other variations will at once be apparent to those skilled in the art upon a reading of this specification. Accordingly the scope of this application should only be limited by the scope of the following appended claims.

I claim:

1. A method of degassing molten metal under vacuum whereby the percentage of included deleterious gases such as hydrogen, nitrogen and oxygen is substantially reduced, said method including the steps of subjecting the surface of a batch of molten metal within a refractory lined molten metal receptacle having a stopper rod seated therein to a vacuum sufiiciently low to degas the molten metal, and simultaneously inducing a mechanical circulation within the receptacle to thereby expose substantially undergassed molten metal from remote areas of the receptacle to the vacuum at the surface by bubbling a purging gas chemically inert with respect to the molten metal upwardly through the refractory lining and thence through the metal from the area immediately adjacent the tip of the receptacle stopper rod to thereby flush away foreign material which would otherwise accumulate in the aforementioned area and maintain the metal fluid around the stopper rod so as to prevent sticking of the rod to the receptacle.

2. The method of vacuum degassing molten metal of claim 1 further characterized in that the vacuum is on the order of about two millimeters of mercury or less.

3. The method of claim 1 further characterized in that a minimum of slag cover is maintained over the surface of the molten metal during a substantial portion of the time the molten metal is exposed to vacuum.

4. The method of vacuum degassing molten metal of claim 2 further characterized in that the surface of the molten metal is substantially uncovered by slag to thereby provide maximum opportunity for the vacuum to act on the molten metal.

5. A method of degassing molten metal under vacuum whereby the percentage of included deleterious gases such as hydrogen, nitrogen and oxygen is substantially reduced, said method including the steps of subjecting the surface of a batch of molten metal within a ladle to a vacuum sufficiently low to degas the metal, and simultaneously inducing a mechanical circulation within the ladle to thereby expose substantially undegassed molten metal from remote areas of the ladle to the vacuum at the surface by bubbling a purging gas chemically inert with respect to the molten metal upwardly through the stopper rod nozzle block adjacent the nose of the stopper rod to thereby flush away foreign material which would otherwise accumulate in the aforementioned area and maintain the metal fluid around the stopper rod so as to prevent sticking of the rod to the receptacle.

6. The method of vacuum degassing molten metal of claim 5 further characterized in that the inert gas is bubbled upwardly from within the depression formed in the nozzle block.

7. The method of claim 5 further characterized in that a minimum slag cover is maintained over the surface of the molten metal during a substantial portion of the time the molten metal is exposed to vacuum.

8. In a method of producing vacuum degassed steel which, in aninitial molten condition, contains deleterious gases and a layer of slag exposed to substantially atmospheric pressure, the steps of applying moderate superheat to the molten steel,

decreasing the quantity of slag on the surface of the molten steel to thereby provide a substantially decreased slag cover, and

subjecting the molten steel and the remaining slag, if

any, to the action of a vacuum of about 4-6 mm. Hg absolute or below for a substantial portion of the time the metal is exposed to vacuum.

9. The method of claim 8 further characterized in that the vacuum is maintained at a level of about two millimeters of mercury or below for a substantial portion of the time the molten steel is exposed to vacuum.

10. The method of claim 8 further characterized in that only a very bare minimum of slag is present at the commencement of degassing.

11. The method of claim S-further characterized in that the molten steel is exposed to a non-explosive gas subsequent to degassing but prior to exposure to the atmosphere.

12. The method of claim 8 further characterized in that the molten steel is agitated during at least a portion of the time it is exposed to the vacuum.

13. The method of claim 8 further including the step of thereafter making additions to the molten steel.

14. A method of degassing molten metal under vacuum whereby the percentage of included deleterious gases such as hydrogen, nitrogen and oxygen is substantially reduced, said method including the steps of preparing the metal for degassing by applying sufficient heat from a suitable source to cause both the metal and its associated slag to be in a molten condition prior to degassing,

removing slag from the surface of the molten metal prior to degassing so that the metal has no more than a thin slag layer thereon at the commencement of degassing,

thereafter subjecting the molten metal, in a refractory lined receptacle having a discharge aperture therein, to a vacuum sufficiently low to elfectively degas the molten metal, and, during at least a portion of the time the molten metal is exposed to the vacuum,

including an agitation within the molten metal, including the discharge aperture area, of a magnitude sufiicient to prevent erratic pouring behavior of the molten metal as it is subsequently discharged through the discharge aperture by bubbling a purging gas through the refractory lining and. thence upwardly through the molten metal.

References Cited UNITED STATES PATENTS 963,652 7/1910 Reynolds -49 X 1,554,368 9/1925 Rackoff et a1. 75-49 2,993,780 7/1961 Allard 7549 3,083,422 4/1963 Finkl 74-4-9 FOREIGN PATENTS 545,895 3/1956 Belgium.

DAVID L. RECK, Primary Examiner. H. TARRING, Assistant Examiner. 

1. A METHOD OF DEGASSING MOLTEN METAL UNDER VACUUM WHEREBY THE PERCENTAGE OF INCLUDED DELETERIOUS GASES MUCH AS HYDROGEN, NITROGEN AND OXYGEN IS SUBSTANTIALLY REDUCED, SAID METHOD INCLUDING THE STEPS OF SUBJECTING THE SURFACE OF A BATCH OF MOLTEN METAL WITHTIN A REFRACTORY LINED MOLTEN METAL RECEPTACLE HAVING A STOPPER ROD SEATED THEREIN TO A VACUUM SUFFICIENTLY LOW TO DEGAS THE MOLTEN METAL, AND SIMULTANEOUSLY INDUCING A MECHANICAL CIRCULATION WITHIN THE RECEPTACLE TO THEREBY EXPOSE SUBSTANTIALLY UNDERGASSED MOLTEN METAL FROM THE REMOTE AREAS OF THE RECEPTACLE OF THE VACUUM AT THE SURFACE BY BUBBLING A PURGING GAS CHEMICALLY INERT WITH RESPECT TO THE MOLTEN METAL UPWARDLY THROUGH THE REFRACTORY LINING AND THENCE THROUGH THE METAL FROM THE AREA IMMEDIATELY ADJACENT THE TIP OF THE RECEPTACLE STOPPER ROD TO THEREBY FLUSH AWAY FOREIGN MATERIAL WHICH WOULD OTHERWISE ACCUMULATE IN THE AFOREMENTIONED AREA AND MAINTAIN THE METAL FLUID AROUND THE STOPPER ROD SO AS TO PREVENT STICKING OF THE ROD TO THE RECEPTACLE. 